This invention relates to methods for monitoring the exhaust gas conversion rate of an exhaust catalyst intended for an internal combustion engine.
Procedures for monitoring the exhaust gas conversion rate of exhaust catalysts are known, for example from U.S. Pat. No. 5,419,122 and German Offenlegungsschrift No. 42 11 092. U.S. Pat. No. 5,419,122 discloses a catalyst monitoring procedure which detects exhaust gas temperatures upstream and downstream of the catalyst. The rate of heat transfer from the exhaust gas to the catalyst material is continually calculated and the catalyst temperature and the rate of change of the catalyst temperature are determined from these calculations.
In addition, the quotient of the rate of heat transfer and the rate of change of the temperature of the catalyst is determined for a specific period of time which is the time from the cold start of the internal combustion engine to the time when the exhaust gas conversion (start-up) temperature of the catalyst is attained. Based on the variation of this quotient over time, a trend is continually determined until the conversion temperature is attained. The time period required to reach the conversion temperature is compared with a threshold value for that time period and, if the threshold value is exceeded, an error signal is produced.
One of the procedures disclosed in Offenlegungsschrift No. 42 11 092 utilizes a temperature estimate for the catalyst which is based on a temperature model. In that procedure, it is assumed for the model that the catalyst is still just usable after reaching the temperature at which the exhaust gas conversion begins and with the heat produced in the conversion. For simulating the temperature behavior of the catalyst, it is initially assumed in that procedure that the heat capacity of the catalyst is known. The amount of heat supplied to the catalyst by the engine is estimated by determining the amount of fuel supplied to the engine and calculating the proportion of the resulting combustion energy which is transferred to the catalyst. In this case, the proportion of combustion energy depends on the engine load condition and the speed of the engine. Consequently, the amount of heat which is supplied to the catalyst is determined during a series of comparatively small time intervals based on the amount of fuel, the load and the speed during each interval. These amounts of heat are summed in order to obtain the total amount of heat which the catalyst receives from the engine from the beginning of the procedure. In addition, the amount of heat dissipated from the catalyst to the environment is estimated. The catalyst temperature estimated using this procedure is subsequently compared with its actual temperature.
The foregoing procedure, which is also intended to be applicable during the driving mode of a vehicle, is utilized only if the catalyst is at approximately ambient temperature at the beginning of the procedure, which can be determined, for example, by comparing the water temperature and the oil temperature of the internal combustion engine with the ambient temperature. From the start of the procedure, a catalyst temperature is estimated with the assistance of a temperature model unit which receives signals corresponding to the engine load, the vehicle speed, fuel injection times, air temperature, air pressure and the octane number of the fuel. This estimated catalyst temperature is compared with a stored comparison temperature. The procedure subsequently determines whether the conversion temperature of the catalyst has already been reached.